Patent Document 1 discloses a pneumatic radial tire as related conventional art of this kind.
In the pneumatic radial tire, at least one main groove extending in the tire-circumferential direction is formed in a portion on the outer side of the tread portion when the tire is fitted to the vehicle. In addition, of all the main grooves thus formed, the one that is closest to the tire equator line is set to have a relatively large width. Thereby the pneumatic radial tire can improve the drainage of the tread portion.
In addition, in the pneumatic radial tire, at least two main grooves extending in the tire-circumferential direction are formed in portions located on the inner side of the tread portion when the tire is fitted to the vehicle. Of all the main grooves thus formed on this side, the one that is closest to the tire equator line is set to have a relatively small width. Thereby, the pneumatic tire can reduce the noise generated in the tread portion.    Patent Document 1: JP-A 2004-90763